This invention relates to controlled energy absorption in crashworthy structures and more specifically aircraft and helicopter type structures made from composite materials.
Early helicopters were constructed with what is generally known as truss structure, i.e. a framework of longerons, running lengthwise, and struts, running up and down and crosswise, and both of which in turn, were strengthened by stringers. Monocoque structure uses a stressed skin outer shell which is strengthened with bulkheads and rings. Most helicopters today use a semimonocoque structure which is a combination of truss and stressed-shell construction. It is a stressed-shell reinforced by longerons and stiffeners for added strength. Any unstiffened plate or shell structure under compressive or shear loading is likely to have its load-carrying capability severely limited by the onset of structural instability. The weight-efficiency of these, structures can generally be enhanced by the addition of an arrangement of stiffening members. Such a structural assembly can be designed to be unbuckled, or the skin elements can be allowed to buckle before the design or ultimate load is reached.
Stiffening members that are continuous will strain with the shell members and hence carry a portion of the load. By aligning the stiffeners along dominate load directions a more efficient structural arrangement is achieved. For this reason, it is common practice to run the stiffeners in a fuselage generally along the length of the structure.
The stiffeners themselves must be supported at intervals by transverse stiffening members to prevent their failure by column buckling. The transverse members serve to maintain the cross-sectional shape in shell structures and also resist crushing loads when bending curvatures are imposed. The entire stiffened structure, of course, must be checked to make sure that general instability does not occur. These stiffeners are generally oriented perpendicular to the stress skin outer shell. The design procedure that has been discussed so far is to accommodate in-flight, landing and take-off loads without any consideration of crash loads.
Modern helicopter design requires a maximum level of crashworthiness protection without adding undue weight or complexity to the air frame. The crashworthiness of a helicopter design is a systems approach in that the air frame, landing gear, stroking seats and fuselage crush zone all cooperate and combine together to provide survival deceleration rates and structural integrity throughout the crash. For instance some parts of the fuselage are designed
to crush and thereby absorb energy while other parts carry the load from major mass items, e.g. the engine and gear box to the crushing structure. This particular invention is directed to the composite fuselage crush zone which must be capable of absorbing a given amount of energy as well as distributing the crushing so as not to induce acceleration effects due to additional pitching and rolling.
Earlier helicopters were made of all metal fuselages and even if the columns or stiffeners did not fail with maximum crushing efficiency the ductility of the metal allowed the various elements of the structure to cooperate and still produce a failure mechanism that was an effective energy absorber. In fact, many state of the art metal fuselages have proven crashworthy in service. However, the structurally efficient materials known as composites which employ organic polymers or resins reinforced with cloth fibers or rovings have very desirable mechanical and physical properties. The graphite composites, for instance, are extremely strong but inherently very brittle and exhibit nonprogressive failure characteristics. Metals absorb energy through plastic deformation but composite materials do not so deform. If not intentionally designed otherwise, failure of a composite vertical structural member resembles a classic Euler instability column failure with very little energy absorption. The secret is to make these structurally efficient materials fail progressively, in compression, and maintain stability throughout the crash for maximum energy absorption. In other words, the compression failure must begin at one end of the column and progress to the other end to get maximum stroke and energy absorption of the failed column. If this cannot be accomplished failure of the composite materials in compression absorbs energy, however, the organic polymer material is friable and when it fails in compression it disintegrates into very small particles, splinter like in nature.
The procedure is to first determine what structure is required to accommodate the flight loads, discussed earlier, then take that structure and change it, where required, to also provide maximum energy absorption in a crash type failure without adding undue weight.
One of the techniques used in the prior art to stabilize the stiffening members for crushing includes the incorporation of web elements to provide external support to stabilize the vertical stiffener as it crushes. The webs must stay attached to the stiffener throughout the crushing event. However, even if attachment is maintained, progressive crushing is not guaranteed but, at best, provides enough stability for some buckling to take place before the structure becomes unstable. Another means of keeping the crushed elements stable is to incorporate a core material as taught in U.S. Pat. No. 4,593,870 to Cronkhite et al. For this method to perform effectively, the core must be sufficiently thick in relationship to the length of the stiffener to ensure that the stiffener will remain stable regardless of where the compressive failures occur. Another method as taught by Wilson et al in U.S. Pat. No. 4,336,868 is to run a continuous filament wound tube used as an anvil to stabilize the structure. In all of these methods, a loss of all or part of the external stabilization significantly reduces the amount of energy that a given structure can absorb. Further, reliability, weight and, in the case of military aircraft, decontamination of these closed structures are factors that must also be taken into consideration.
It is an object of this invention to provide open stiffening elements from structurally efficient composite materials which fail progressively, maintaining stability throughout the crash stroke for maximum energy absorption without relying on other stabilizing elements that could come unattached during crushing and add substantial weight.
It is a further object of this invention to provide predictable and controllable energy absorption characteristics to conventional structural stiffener elements commonly used in the aircraft industry such as T, Z, C, hat and cruciform sections made from all fiberous reinforced composites.